Many state and federal agencies require that all products produced with ozone-depleting substances such as solvents be labeled as such. These same agencies also will not allow new adhesive spray installations to be installed without very expensive solvent burners to clean recirculate solvent-laden air.
One solution to this problem is to switch from solvent-based adhesives to water-based adhesives which contain little or no solvents. When a water-based adhesive is applied by conventional or high volume, low pressure (HVLP) guns, tremendous processing problems result such as longer "tack time." This is also called "green strength."
With solvent-based adhesives, an operator could process his/her parts very quickly due to quick solvent evaporation. With water-based adhesives, there is a significant waiting time between spray and process. This is due to the water which must be evaporated. Most water-based adhesives are 30% to 50% water by weight. For example, an 8 lb. gallon of water-based adhesive contains 21/2 lbs. to 4 lbs. of water. A substantial portion of the water must be evaporated before "tack" is achieved.
Operations such as repairing foam cushions, sticking pieces of foam together, applying fabric, etc. can easily take 3 to 5 times longer with water-based adhesive if no assist is given to the process. Many plants have attempted to install special heating units in order to dry the water-based adhesive and to trap water under the surface. Even with additional ovens, processing time is increased dramatically. Many mechanical means have been attempted. Hot air guns similar to hair dryers have been used after spraying water-based adhesive with a conventional gun. This doubles operator application time.
All of these measures require more capital investment by increasing the length of processing lines, adding more ovens and establishing accumulating areas for the parts. More operators are needed as well to keep up with line speeds.
U.S. Pat. No. 4,761,299 discloses a method and apparatus for spray coating an article in a coating zone with a liquid coating material, such as paint, wherein air is supplied to the spray head of an air spray gun at an atomizing air flow rate in excess of 5 CFM and at a delivery pressure of less than 15 psi to atomize the liquid coating material. A turbine unit filters and heats the air so that the air has a temperature in excess of 70.degree. F. at the spray head. However, this temperature is not high enough to properly dry a water-based adhesive.
Japanese Patent Document JA 9,042,032 discloses, in its translation, a hot air atomizer for helping to disperse suspended liquid such as a watery liquid. The atomizer is a non-air atomizing rotary bell or disk. Material is atomized by electrostatic centrifugal means and air is used for shaping only. The hot air is used to lower viscosity of thick materials which have a tendency to block fluid dispensing openings. Air temperature is 100.degree. to 120.degree. F.
U.S. Pat. No. 4,667,084 discloses an adhesive spray gun system that uses an electrically heated hot air system for atomizing a melted adhesive. A heater hose heats both hot melt adhesive and atomizing air. A hot melt adhesive is 100% solids in block form. It is then melted into a thick liquid for application. The purpose of the hot air is to keep the hot melt from drying in the hose or on the gun.
U.S. Pat. No. 5,102,484 discloses an adhesive spray system using a hot gas, such as hot air, to keep the adhesive soft prior to working. There is no mention of atomizing hot melt viscous material. Hot gas at the applicator head keeps viscosity down and assist in swirling the patterns but the hot gas does not mix with the material.
U.S. Pat. No. 4,964,569 discloses a warm air spray system for preventing the formation of condensation in its supply and return lines. Warm air under high pressure is used in "any desired spray device." The purpose is to reduce condensation in atomizing lines.
U.S. Pat. No. 4,669,661 discloses a hot melt glue sprayer that uses heated air to ensure that the hot melt sprays efficiently and accurately. A hot melt glue gun uses high pressure hot air to keep glue soft and applicable.
U.S. Pat. No. 3,776,462 discloses a sprayer for molten metal that uses heated air under pressure to atomize the spray. Hot air heats metal to keep it molten. High pressure air "propels atomized particles at high velocity onto the surface to be coated."
U.S. Pat. No. 4,785,996 discloses an adhesive spray system that uses a plunger mechanism for allowing adhesive to be released into a spray cavity. High pressure cold air is used to divert "bead of extruded hot melt adhesive."
U.S. Pat. No. 3,796,376 discloses a spray gun that has a trigger actuated plunger to control the flow of liquid. High pressure cold air is emitted through a special valve/plunger mechanism in the handle of the gun.
U.S. Pat. No. 5,076,469 discloses a spray gun system that uses a heated gas to ensure a better application of hot melt adhesives or the like. The gun uses an electrical resistance heater to keep hot melt adhesive molten. Compressed gas (air) is heated under high pressure.
U.S. Pat. No. 4,925,101 discloses a wax spray gun that uses an operating plunger mounted with an air valve to allow atomizing air to be admitted to the wax. Cold atomizing air is used under high pressure.